Thank you to both BALIX and PPRuNe Radar. This is something that's been irritating and puzzling me for weeks. I've been pondering asking about it here from time to time, without ever quite getting around to it. Now I have the answer!
I think, in my case, this is happening to me because I'm using additional functions on the keyboard that I've never used regularly before, such as Alt+Tab for switching between windows - and my fat fingers are getting in the way.